In a detector, a pixel represents the elementary sensitive element of the detector. Each pixel converts an electromagnetic radiation, or a flow of charges for a photoconductor, to which it is subjected into an electrical signal. The electrical signals derived from the different pixels are collected during a matrix-reading phase and are then digitized in such a way that they can be processed and stored to form an image. The pixels are often formed from a photosensitive area delivering a current of electric charges as a function of the flow of photons which they receive, and from an electronic circuit for processing this current. The photosensitive area generally includes a photosensitive element or photodetector which may, for example, be a photodiode, a photoresistor or a phototransistor. Large-dimension photosensitive matrices exist which may comprise several million pixels.
A radiation detector can be used for the imaging of ionizing radiation, and notably X or γ radiation, in the medical sector, for example for the detection of radiological images, or for non-destructive testing in the industrial sector. The photosensitive elements enable detection of visible or near-visible electromagnetic radiation. These elements are not sensitive or are barely sensitive to the radiation incident on the detector. A radiation converter known as a scintillator is then often used which converts the incident radiation, for example an X-radiation, or a radiation in a band of wavelengths to which the photosensitive elements present in the pixels are sensitive. An alternative consists in implementing the photosensitive element in a different material, referred to as a photoconductor, carrying out the direct conversion of the X-radiation into electric charges. This occurs, for example, in the case of matrices in which a first pixelized cadmium Telluride (CdTe) substrate is connected pixel-by-pixel to a CMOS read circuit which therefore no longer has the detection function.
It is known to implement an electronic processing circuit by means of a voltage follower enabling the reading of the charges accumulated in the photosensitive element, said charges forming a photosignal. A current source provides the power supply of the pixel during its reading.
In order to improve the quality of the useful image and reduce the level of noise in the useful image, a reading of each of the pixels of the matrix can be carried out by means of correlated double sampling (CDS), well known in the English-language literature. This method consists in performing two successive operations of reading the same pixel, the first, without the photosignal, immediately after a reset, the second, with the photosignal, with no reset between these two readings. A subtraction of the levels obtained in each of the read operations allows the level of noise linked to the pixel reset to be eliminated. The temporal proximity of the two read operations allows some temperature offsets of the detector to be eliminated.
A major disadvantage of the correlated double sampling reading is the prolongation of the detector reading time. In fact, it is necessary for a row of the matrix to perform the two read operations and also the reset operation before beginning the reading of the next row. Assuming that the read and reset operations each occupy the same time period, the complete correlated double sampling reading of the matrix requires three times more time than a simple reading without double sampling.